A critical stage in early breast tumorigenesis is the emergence of preneoplastic cells that carry key tumor drivers but remain in a latent state similar to normal quiescent cells. Such latent preneoplastic cells have been reported in precancerous lesions and tissues surrounding tumors, and are implicated as precursors for primary and recurrent or second field tumors. Despite advances in detection, little is known about the properties of preneoplastic cells to predict or prevent tumor formation. Our long-term goal is to understand the cellular controls in preneoplastic cells and to exploit these controls for developing novel prognostic markers and tumor prevention strategies. The quiescence and outgrowth of latent preneoplastic cells has important implications in driving tumor formation. Here, we propose to investigate this proliferative control in preneoplastic breast cells with hyperactivated AKT signaling, a major tumor driver found in early lesions of the breast and many other epithelial tissues. A major barrier to studying latent preneoplastic cells at a quiescent state is the lack of experimental models. To overcome this, we recently devised specialized organoid and in vivo models of quiescent preneoplastic cells by inducing oncogenic alterations in growth-arrested 3D organoids of non- transformed human mammary cells and by direct engineering cells in intact mouse mammary glands. Using these models, we recently reported that aberrant AKT activation in preneoplastic breast cells induces a primed quiescent cell state that maintains growth arrest under normal conditions, but promotes selective proliferation in tumor-promoting microenvironments. To gain insight into this cell state, our preliminary studies revealed a unique cell cycle program with concurrent upregulation of cell cycle inhibitor, p57Kip2 (p57), and promoter, cyclin D1. This increase in p57 protein abundance is also evidence in AKT-induced preneoplastic cells in mice. Notably, we found that attenuating this AKT-induced p57 upregulation is sufficient to drive these quiescent preneoplastic cells to proliferate and that downregulation of p57 protein level is linked to proliferative outgrowth in tumor-promoting microenvironments. These findings lead us to hypothesize that a p57-mediated cell cycle program controls the maintenance and outgrowth of a primed quiescent state induced by AKT in latent breast preneoplastic cells. We propose three aims to 1) define the molecular identity and induction mechanism of this AKT-induced cell cycle program, 2) elucidate the regulatory role of p57 in the proliferative control of these quiescent preneoplastic cells, and 3) determine the role of p57 in mediating mammary preneoplastic development in mice. These studies will reveal the functional role and the underlying mechanism, as well as provide insights into the prognostic value, of p57 in the progression of preneoplastic breast lesions with aberrant AKT activation. More broadly, our studies will advance our understanding in the control of preneoplastic development at the cellular level, which will provide the mechanistic bases for developing novel prognostic markers and tumor prevention strategies for individuals with increased breast cancer risk.